Understanding the roles of the microbiome in autoimmune rheumatic diseases

Abstract The gut microbiome represents a potential promising therapeutic target for autoimmune diseases. This review summarizes the current knowledge on the links between the gut microbiome and several autoimmune rheumatic diseases including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE) spondyloarthropathies (SpA), Sjogren’s syndrome (SS), and systemic sclerosis (SSc). Evidence from studies of RA and SLE patients suggests that alterations in the gut microbiome composition and function contribute to disease development and progression through increased gut permeability, with microbes and microbial metabolites driving an excessive systemic activation of the immune system. Also, there is growing evidence that gut dysbiosis and subsequent immune cell activation may contribute to disease pathogenesis in SpA and SS. For SSc, there are fewer, but these are still informative, reports on alterations in the gut microbiome. In general, the complex interplay between the microbiome and the immune system is still not fully understood. Here we discuss the current knowledge of the link between the gut microbiome and autoimmune rheumatic diseases, highlighting potentially fertile areas for future research and make considerations on the potential benefits of strategies that restore gut microbiome homeostasis.


Introduction
Autoimmune rheumatic diseases are a diverse and heteroge neous group of disorders that affect tens of millions of people worldwide [1] and are characterized by the immune activation with clonal expansions of lymphocytes that target the host's own tissues and organs.While there has been an uneven improvement in outcomes for several of these autoimmune diseases, their etiology and pathogenesis are not yet fully un derstood.The microbiome, which includes the diverse collec tion of microorganisms that reside on and within the human body, [2] has emerged in recent years as a potential player that can shape immune responses and contribute to the patho genesis of various autoimmune diseases.This review focus es on emerging advancements that link the gut microbiota to immune dysregulation in rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), spondyloarthropathies (SpA), Sjogren's syndrome (SS), and systemic sclerosis (SSc).
The microbiomehost relationship represents a complex in terplay between the host and its resident microbial communi ties whereby gut microbes play a crucial role in maintaining gut barrier integrity and immune homeostasis while prevent ing the expansion and mucosal invasion of potentially harm ful species. [3,4]While some studies have shown a correlation between gut microbiota imbalances (also termed dysbiosis) and inflammatory and autoimmune conditions, [5] recent re ports have suggested that the overgrowth, or bloom, of typi cally harmless gut bacteria can perturb the gut epithelial bar rier and trigger immune responses to microbial antigens.This can lead to the onset or flare-up of autoimmune rheumatic diseases. [6]In one example, the gutjoint hypothesis proposes that microbial dysregulation in the gut can lead to immune cell migration to joints, driving inflammatory responses. [7] patients with active RA and SLE, studies have shown that dysbiosis of gut microbiota community composition in as sociation with altered metabolic pathways.Dysbiosis in the gut has also been associated with increased intestinal per meability in RA, SLE, SpA, and other autoimmune diseases, as discussed below. [8,9]Specifically, it has been suggested, at least in some cases, that a translocation of bacterial products into the draining lymph nodes provides microbial exposure to the immune system for the exacerbation of pathologic inflam matory responses (Figure 1). [10]o define a paradigm for the gut microbiome-host relation ship, one can consider the billions of bacteria in the gut as an ocean-like ecosystem-many species residing within a specific niche, local microenvironments and microbiota subcommuni ties.The dynamic forces that govern the relative abundance of the multitude of diverse species are in part affected by the downstream flow of gut contents.The mucus layer gener ated by gut epithelial cells contributes to a functional barrier.High local levels of secreted dimeric IgA produced by mu cosal plasma cells recognize certain species and may has ten their clearance, impede their proliferation, and decrease their interactions with intestinal epithelial cells.The gut bar rier microenvironment and its adjacent mucosal layer func tion to rapidly mobilize cellular defenders to avoid infectious disease, targeting pathogens from crossing the barrier.When there is a physiologic balance between local commensal spe cies, both pathogenic bacteria and otherwise commensal pathobionts are retained within the gut.Yet, the intestinal epi thelial lining is not impenetrable.The expansions, or blooms, of select bacterial species-or local inflammatory responsescan increase intestinal permeability through receptormedi ated mechanisms affected by apical epithelial production of zonulin. [8]Specifically, dysfunction in the intestinal barrier is reported to occur in the absence of substantial tissue injury by tight regulation of the zonulin pathway.Zonulin modulates the intracellular formation of tight junctions between intestinal epithelial cells lining the gastrointestinal (GI) tract, which play a crucial role in maintaining gut barrier integrity by impeding the passage of molecules from the intestinal lumen into drain ing lymphatics and subsequently into the bloodstream.In this setting, intestinal bacteria, and their inflammatory products, can traverse the mucus and intestinal epithelial barrier, fa voring microbialimmune interactions.If host characteristics promote gut permeability, including through zonulinmediated tight junction modifications that affect the gut epithelium bar rier, then a lower amount of microbiota is sufficient to cross the gut epithelium.The increased gut permeability, along with intestinal cell death, can result in (auto) antigenic stimula tion and enhanced immune signaling that can contribute to breaches in peripheral immune tolerance.
Herein, we begin by considering the historical context of the intersecting fields of microbiology and rheumatology, and then discuss the mechanistic links between commensal mi crobial dysbiosis and RA, SLE, SpA, SS and SSc.We will also discuss the proposed molecular mimicry mechanisms in this burgeoning field.

A Historical Perspective
Autoimmune diseases now affect an estimated 5%-10% of the western populations. [1,11]Though inherited genetic fac tors certainly contribute to susceptibility, environmental fac tors are also postulated to function as triggers for disease initiation and later flares.Gut microbiota communities, which consist of trillions of microorganisms residing in the GI tract, [2] play essential roles in the production of vitamins, nutrients availability, and general immune regulation and homeostasis.Moreover, there is increasing recognition of the connection between infection and dysbiosis with gut microbiota commu nities in autoimmune pathogenesis.
In the earliest formulation of the theory of autoimmunity, which dates to the early 1900s, the French immunologist Paul Ehrlich proposed the concept of "horror autotoxicus"(fear of selftoxicity), which refers to the ability of the immune sys tem to distinguish between self and nonself. [12]Since then, steady progress has been made to understand the features and mechanisms responsible for autoimmune pathogenesis, including the molecular mechanisms by which the immune system balances receptor mediated processes that enable distinguishing self from nonself.Especially in the context of infectious disease, several studies have provided support for the molecular mimicry hypothesis, including those focused on rheumatic heart disease.Although controversies remain, the group A strain of the opportunistic commensal pathogenic bacterium, Streptococcus pyogenes, expresses a carbohy drate epitope, NacetylbetaDglucosamine (GlcNAc), to which patients with acute rheumatic fever make IgG2 anti bodies, which in turn also target heart valve tissue, contribut ing to rheumatic heart disease. [13,14]Similarly, a region within Epstein-Barr nuclear antigen 1 (EBNA1), an immunodomi nant protein of Epstein-Barr virus-a common human patho gen that in most cases does not cause clinical symptoms in infected individuals was shown to have homology with an epitope within human myelin and could induce autoimmune encephalitis in a murine model system of multiple sclerosis. [15]wever, molecular mimicry may only represent a possibil ity.[18] Enhanced immune signal ing and inflammation, [19] lymphocyte activation, [20] polyclonal Ig secretion, [21] and antigen receptormediated lymphocyte apoptosis, [22] have all been observed in the presence of gut microbiota dysbiosis.The gut barrier integrity is also influ enced by the gut microbiome that can modulate immune re sponses by producing metabolites, such as the small chain fatty acid butyrate, [22] although direct mechanistic links have yet to be proven.

Rheumatoid Arthritis
RA is a chronic systemic autoimmune disease characterized by synovial inflammation, cartilage destruction, and bone ero sion. [23]While it has long been known that the pathogenesis of RA involves the interaction between genetic and environmen tal factors, more recent mounting evidence has also shown Patients with RA exhibit an altered gut and oral microbiota composition and function, with a reduction in beneficial bacterial species and increased pro-inflammatory bacteria.Specifically, taxon-level analysis identified an expansion of rare taxa, Actinobacteria, with a decrease in abundant taxa in RA patients compared to controls, and found three genera, Collinsella, Eggerthella and Faecalibacterium, to be associ ated with RA. [24] Additionally, RA patients with the HLA-DRB1 allele epitope, and especially those who become anti citrul linated protein antibody (ACPA) positive, are atrisk of RA.These individuals often have dysbiotic subgingival microbi omes associated with an increased abundance of the com mensal, Porphyromonas gingivalis, compared to controls. [25]man observational studies and murine models have pro vided valuable insights into the gutjoint axis in RA. [26] At dis ease onset, RA patients had increased intestinal microbes including Prevotella copri compared to healthy controls, and the expansion of these rare lineage intestinal microbes as sociated with an increase in the proinflammatory cytokine in terleukin17 (IL17). [27]These findings correlated with lowered diversity of Clostridia, Lachnospiraceae, and Bacteroides in the RA gut. [27]Beyond the gut microbiome, the composition of oral microbiota communities was altered in patients with RA.Particularly, P. gingivalis has been considered as an extra articular trigger for RA and other autoimmune diseases. [28]eriodontal disease has long been known to be associated with an increased risk of RA, [25] but it remains controversial whether this could be a cause or a consequence of RA dis ease.Specifically, RA patients with positive anti-citrullinated protein antigen (ACPA) antibodies exhibit a higher abun dance of P. gingivalis in their oral microbiota, and P. gingivalis expresses peptidylarginine deiminiase (PAD), which can convert arginine to citrulline in host proteins, generating citrullinated antigens. [29]Aggregatibacter actinomycetem comitans, [30] which is also linked to periodontitis, is another oral bacterial microbe which shares the ability to generate citrullinated autoantigens.More recently, a human gut com mensal called Subdoligranulum didolesgii, was implicated in RA pathogenesis.This putative pathobiont, identified with the use of RA patient serum for antibody cloning, was sub sequently found in murine gut colonization models to cause synovitis and deposition of complement and antibodies, even in the absence of an experimental adjuvant trigger. [31]G mice, that spontaneously develop arthritis, when re derived as germfree were protected from developing ex perimental arthritis, while colonization of germfree mice with segmented filamentous bacteria (SFB) developed inflam matory polyarthritis. [32]Also, colonizing SKG mice in specific pathogen free (SPF) conditions with fresh human feces from RA patients with high abundance of Prevotellaceae promoted the development of inflammatory polyarthritis. [33]crobial effects may be relevant for understanding whether a RA patient responds to treatment with diseasemodifying antirheumatic drugs (DMARDs).For example, there is evi dence that gut dysbiosis correlates with worse clinical re sponses to treatment with the most commonly used DMARD, methotrexate. [34,35]Moreover, the treatment of RA with a tumor necrosis factor (TNF)a inhibitor has been reported to also partially restore the balance within a patient's gut commensal community, which supports the notion that systemic inflam mation in RA may itself be a driver for gut dysbiosis. [36] RA, it has also been proposed that there may be clinical benefits that can be attained by targeting gut dysbiosis through probiotic supplementation that introduces certain gut spe cies.Monotherapy with ingestion of pathobiont Tripterygium wilfordii Hook F has been demonstrated to be noninferior to methotrexate monotherapy in controlling disease activity in a small, although nonblinded, study of DMARDnaïve rheu matoid arthritis patients. [37]Additionally, clinical trials that as sessed the use of probiotics in RA patients found that probiotic supplementation (with Lactobacillus acidophilus, Lactobacillus casei, and Bifidobacterium bifidum) can improve clinical symp toms and markers of inflammation. [38,39]Probiotic supplemen tation also significantly decreased insulin resistance and im proved lipid metabolism in RA patients. [38]However, the use of probiotics in RA needs further validation before it can be adopted for general use.
Another approach with potential benefits is the use of shortchain fatty acids (SCFAs), which are metabolites produced by gut bacteria during fermentation of dietary fiber that is other wise indigestible.In patients with RA, gut dysbiosis can alter the composition of gut microbiota, leading to changes in the production of SCFAs.SCFA levels may themselves play a role in the pathogenesis of RA by directly modulating immune cell function and associated inflammatory responses.The most prominent SCFAs are acetate, propionate, and butyr ate, which have postulated immunomodulatory effects in RA patients.
SCFAs have been reported to inhibit the onset of experimen tal arthritis, and serum butyrate levels were found to be de pressed at times before the onset of arthritis. [40]Conversely, in mice the administration of SCFAs improves the severity of arthritis, in part through the regulation of B cell differen tiation mediated by free fatty acif type 2 receptor (FFA2), a G protein coupled receptor for fatty acids. [41]In a prospective cohort study of individuals with an increased risk for devel oping RA, the subjects who progressed to clinical arthritis had lower serum levels of total SCFAs, and especially butyr ate, and acetate, at baseline compared to those who did not progress to overt disease. [42]These early findings support the need for further investigations on the role of SCFAs in RA.Other studies of dietary interventions in RA patients showed that a low-calorie, Mediterranean diet significantly changed the metabolic pathways in the GI tract of RA patients, includ ing the metabolism of amino acids and lipids with associated microbiome differences between RA responders and nonresponders to treatment. [43,44]e abovedescribed studies suggest that future therapeutic considerations for RA may include dietary modification, and the direct targeting of microbial dysbiosis, which by them selves have limited potency, but which might be utilized in conjunction with conventional RA treatments.However, de velopment of the most effective methodology to target dys biosis will require further investigation.

Systemic Lupus Erythematosus, murine models and autoantigens
Murine lupus models have suggested the potential relevance of certain mechanisms triggered by gut dysbiosis.Mice gut colonized with segmented filamentous bacteria displayed worsening of lupus nephritis which was associated with in creased CD206 + macrophage infiltration. [45]Enterococcus gallinarum, a human gut commensal bacterium, when in troduced into the mouse GI tract of germfree mice induced antidsDNA antibodies.Of note, E. gallinarum translocation was documented, as this microbe was be cultured in these colonized mice from blood, lymph fluid, and liver. [46,47]veral studies have suggested that bacterial antigens and metabolites can induce production of autoantibodies in SLE patients.These may include DNAbinding amyloids (curli), [48] bacterial lipopolysaccharides, lipoglycans, teichoic acids, and other bacterial products that can activate the innate immune system via tolllike receptors (TLR) and other patternrecog nition receptors on immune cells. [49]Furthermore, microbeim mune cell interactions may trigger neutrophils to undergo cell death.One such pathway may result in release of neutrophil extracellular traps, as neutrophiles undergo NETosis that en hance immune recognition of commensal antigens that rep resent orthologs of selfantigens.In one compelling example, Ro60, which is an RNA binding protein that is a common target of autoantibody responses in patients with lupus and Sjogren's syndrome, was shown to also become recognized by specific T cells in lupusprone mice colonized with commensal bacteria that naturally express Ro60 orthologs. [50,51]Notably, depletion of Ro60 orthologexpressing bacteria reduced the in vivo rep resentation of activated Ro60-specific T cells. [50]

Studies of the gut microbiome in cohorts of SLE patients
A number of independent crosssectional (i.][54][55][56][57][58] While the gut microbiome in health is dominated by anaerobic species, SLE patients have a decrease in anaerobes, Firmicutes and Bacteroidetes, and an increase in Proteobacteria aerobic species that may bet ter compete in an inflamed host. [54]Studies of fecal samples from SLE patients with active disease have also shown lower diversity and richness compared to healthy controls. [54]Notably, Azzouz et al. documented a decrease in microbiota alpha diversity was inversely correlated with disease severity as measured by the SLE disease activity index (SLEDAI). [52]mpaired intestinal barrier function and associated translo cation of microbiota and metabolic products has provided independent evidence that imbalances in the gut micro biota may contribute to SLE disease pathogenesis. [6,59]ndependent reports on cohorts from diverse geographic ar eas have implicated expansions of the obligate anaerobic commensal, Ruminococcus (blautia) gnavus of the family Lachnospiraceae, in patients with active SLE. [52,60]In a cross sectional study of a cohort in New York City, compared to low stable gut abundance of R. gnavus at a mean 0.1 %, R. gnavus was present at five-fold higher levels in SLE patients, and abundance correlated with the level of lupus disease activity.Highest levels were documented in patients with active renal involvement (i.e., lupus nephritis).Notably, patients with ac tive lupus nephritis displayed the highest levels of serum IgG antibodies to a R. gnavus strain-specific antigen, which rep resents a cell wallassociated lipoglycan [52,61] supporting that gut leak of this antigen is present in affected lupus patients.
In the largest crosssectional study described to date, in a report from Peking University, examinations of 117 untreated SLE patients documented that dysbiosis was common and that, expansions of R. gnavus were the most distinct feature identifying patients with active lupus nephritis. [53,62]Serum antibody responses to the lipoglycan produced by some R. gnavus strains have also been correlated with active lupus nephritis in a cohort that included patients with new onset SLE studied at the Karolinska Institute in Sweden. [61]Taken together, these studies indicate that R. gnavus expansions arise in active lupus patients on three continents.
To directly investigate the pathogenic potential of R. gnavus gut colonization, a number of strains have been iso lated from fecal samples from patients with active lupus nephritis. [63]Analysis of the genome of these strains docu mented the presence of a number of genes believed to ab sent in strains from healthy individuals, but which adapt the strain for survival and imbue ecological competitive advan tages in a host with systemic inflammation. [64]Gut coloniza tion with strains from lupus patients induced dramatically in creased gut permeability, and induced systemic production of antilipoglycan antibodies as well as antiDNA autoanti bodies, [60] but this did not occur following colonization with a strain isolated from a healthy individual.While antilipoglycan antibodies naturally arise in the absence of previous immuni zation, which therefore may be considered a form of natural antibodies, [63] the pathogenic potential of some clones of anti DNA antibodies has been well established. [65]Future studies will be needed to determine if the novel lipoglycan produced by some R. gnavus strains can be utilized as a biomarker for lupus nlupus (LN) disease.
In the first report of a longitudinal study of patients with SLE, fundamental abnormalities in the stability of gut communities in these patients were documented. [64]In health, the composi tion of the gut microbiota community is typically stable over time, representing a dynamic equilibrium that is maintained by currently uncertain mechanisms, In contrast, the overall composition of the gut microbiota in patients with SLE were unstable, and drifted over time, which may indicate that they are particularly vulnerable to disruptions caused by insults such as intercurrent minor infections, food additives in pro cessed food, or perhaps even antibiotics. [64]Hence, the gut microbiome in SLE patients appears to lack resilience and generally unable to return to a preinsult state, [66] favoring the dysbiosis of the gut microbiome. [64]It is therefore speculated that this can lead to increased gut permeability and sec ondary systemic inflammation that may stoke and worsen selfperpetuating pathways of autoimmune pathogenesis (Figure 1).These longitudinal studies also demonstrated in more than 40% of a small cohort, flares of lupus nephritis occurred at time of blooms of R. gnavus, which were 20-90 fold greater than the gut abundance in healthy individuals.While ephem eral blooms of two other anaerobic commensal species were also documented, these other species were not associated with disease flares.Overall, these studies suggest that dys biosis of the gut microbiota, associated with unstable gut communities, and blooms of lipoglycanassociated R. gnavus strains, may be associated with breaches of immune tol erance, promotion of autoantibody production, and autoim mune renal disease flares.

Spondyloarthropathies
Spondyloarthropathies (SpA) include ankylosing spondylitis (AS), psoriatic arthritis (PsA), reactive arthritis, and nonra diographic SpA. [67]Despite considerable progress in under standing the pathogenesis of SpA, the exact etiology driving initial disease development and subsequent flares remains unknown.Recent evidence suggests that dysbiosis of the gut microbiome may be a contributing player. [68][71] As in SLE and RA, increased gut permeability correlating with elevated levels of zonulin and in creased intestinal permeability have been identified in patients

. Conceptual Depiction of Dysbiosis and Microbiota Blooms Represented As Waves. A healthy gut microbiota includes a complex community with thousands or more distinct species that occupy anatomic and metabolic niches and a range of autoimmune diseases correlate with microbiota imbalancesalso termed dysbiosis. In our studies, patients with lupus nephritis, especially the most severe cases, can develop expansions of single species and strains, also termed blooms. (A) A healthy gut microbiota community includes a dynamic equilibrium that is resilient following stressors such as nutritional and viral infections that generally reset to the preexisting balance. (B) Dysbiosis can be associated with inflammatory conditions and reflects shifts in the relative abundance of bacteria, reflecting metabolomic changes with some expansion of gut microbiota. (C)
Severe dysbiosis can be associated with blooms that include large expansions of specific microbial species (e. g.R. gnavus in SLE) with a community diversity that is not resilient to stressors, though it is uncertain if blooms are causal in autoimmune diseases or reflect inflammation secondary to patients' underlying systemic disease states.Increased gut epithelial permeability secondary to host inflammatory and/ or microbial metabolomic changes also occur.These depictions are oversimplifications representing the (A) balance vs. B) dysbiotic imbalances that occur in many patients with inflammatory and autoimmune diseases C) blooms of individual potential pathobionts, in the highly complex communities that reside within the human intestine.with AS. [72] Murine models suggest that the effect of gut dysbio sis on zonulin mediated regulation of intestinal permeability is more pronounced in females. [60]This unexpected finding could in part explain why many autoimmune diseases, including RA and SLE, predominantly affect women.
As for AS, rats made transgenic for the human antigen pre sentation gene, HLAB27, developed arthritis, characteristic spinal as well as skin and nail changes, and bowel involve ment however, when these rats rederived in a germfree environment, disease did not develop. [7375]In patients with AS, ileal biopsies showed an increased expression of zonu lin, compared to control patients. [68]Though the associated mechanisms have not been well defined, HLAB27 clearly is a disease susceptibility factor, and altered gut permeability ap pears to be involved in pathogenesis.Notably, in one report, patients with Crohn's disease and the extraintestinal mani festation of SpA had significant gut expansions of R. gnavus compared to healthy controls. [76]

Sjogren's Syndrome
Recent studies in patients with SS have demonstrated that the oral microbiome, [77] as well as the gut microbiome, [78] sig nificantly differ from that in healthy individuals.Further, in SS patients, correlative evidence suggests a pathogenic influ ence of certain microbial species within the gut, ocular, and oral microbiota communities, which highlights the now termed gutocularoral axis. [77]SS patients are also reported to have an increase abundance of Actinomyces and Lactobacillus in both stool and oral samples, [79] which implicated these taxa as potential pathobionts.This suggests the possibility that oral bacteria or their products may traverse buccal epithelial cells through local defects in the epithelial barrier.Furthermore, T cell epitope mimicry between SS antigen A (SSA) /Ro60 and various bacteria have been postulated as disease driv ers. [80]In an independent crosssectional study of human SS saliva, based on 16S rRNA amplimer sequencing taxonomic distribution in these communities were documented.These studies have identified four genera of bacteria, Bifidobacterium, Lactobacillus, Dialister and Leptotrichia as significantly different in abundance in SS patients compared to healthy controls. [81]

Systemic Sclerosis
There is also emerging evidence of gut microbial dysbiosis in patients with SSc. [82,83]In crosssectional studies of gut mi crobiome communities in patients with SSc, relatively subtle differences in the microbial diversity have been described.As in SS, there were no significant differences in community alphadiversity in SSc and healthy control subjects.However, on subgroup analysis a decrease in alpha diversity was found in SSc patients with GI symptoms as compared to those without GI symptoms and healthy controls. [84]An increase in gut abundance of the aerobic species, Escherichia coli, and a decrease in gut abundance of beneficial bacteria, such as Bifidobacterium and Lactobacillus, have been observed in SSc patients. [85,86]In a small doubleblinded, placebocon trolled trial, SSc patients that received a predetermined com bination of probiotics had increased stool microbiota alpha diversity and had symptom improvement, specifically for GI reflux, although overall gastrointestinal tract symptom ques tionnaires were not significant difference compared to place bo. [87]Though early stage, these encouraging results suggest that further mechanistic studies are merited.

Perspective
Advances in the still young field of microbiome research have shed light on the intricate relationships between the gut mi crobiota and rheumatic diseases.Research progress has provided new avenues for understanding the pathogenesis and treatment options for these complex conditions that are associated with great morbidity and disability, and at times early mortality.The studies summarized in this review sug gest there is a significant impact of microbial dysbiosis on disease development and progression.However, future re search must emphasize longitudinal studies that simultane ously track changes in the microbiome and immune system over time with repeat sampling, investigate microbial metabo lites produced in the gut and their affect immune tolerance, as well as address how medications and diet can alter the com position of gut microbial species in these patients.Beyond this, microbial network analysis could further characterize the networks of species that dynamically shift during the course of disease.Such studies will better define the specific roles of many different individual candidate pathobionts.
These studies suggest that in some patients, specific microbial species, associated with disease activity and autoantibody pro duction, have identifiable antigens that can serve as valuable di agnostic and prognostic tools.Further investigations are needed to elucidate the mechanisms by which gut microbiota influence rheumatic diseases, including the potential involvement of gut barrier dysfunction, immune dysregulation, and molecular mim icry.Looking ahead, personalized microbiomebased therapies for targeted interventions may be utilized to restore microbial re silience and thereby also promote immune balance in patients with rheumatic diseases.By harnessing the power of the gut microbiota, future research endeavors can revolutionize our un derstanding and treatment of rheumatic diseases.

Conclusion
As a resource for the reader, we have highlighted example key reports on the microbiome in patients with rheumatic autoimmune diseases (Table 1), with apologies to the au thors whose work we have not included.Intervention studies are currently in progress while others are still in the planning stages, with an overarching goal of understanding whether normalization of gut dysbiosis by itself, or through the tar geting of specific bacterial strains, can ameliorate symptoms and/or improve disease manifestations.Such therapies may need to be personalized and tailored according to microbi ome of individual patients, which we now know is affected by diet, and genetic background.
Crosssectional studies have been a good starting point but have limitations as such study methods cannot elucidate the timedependent relationships between gut microbiome and an individual's host immune system.Future longitudinal studies can assess the dynamic changes of multiple species over time and evaluate the relative resilience of the overall microbiota ecosystem within the patients' gut as compared to healthy individuals.Candidate microbial pathobiont spe cies may also associate with diseaseassociated changes within microbiota communities.Such changes can be con ceptualized as perturbations in a complex pool of microbial organisms, in which the most severe alterations are associ ated with blooms of individual pathobionts (Figure 1).Such shifts are also associated with functional breaches of the gut barrier that can induce inflammation that favor the competi tive expansion of certain species and strains, including those implicated in pathogenesis, representing a feed-forward influ ence on autoimmune pathogenesis.A better understanding of the complex interplay of these altered communities with the host immune system can lead to strategies of intervention that are, at present, still in their infancy, yet, as indicated in small animal studies, have great potential.
In conclusion, while there is compelling evidence for the influ ence of altered microbiota communities in autoimmune rheu matic diseases, more research is still needed to further ex plore microbial blooms and the specific actors in the breach of the gut epithelial integrity, to ultimately develop new poten tial therapeutic targets.

Figure 1
Figure 1.Conceptual Depiction of Dysbiosis and Microbiota BloomsRepresented As Waves.A healthy gut microbiota includes a complex community with thousands or more distinct species that occupy anatomic and metabolic niches and a range of autoimmune diseases correlate with microbiota imbalancesalso termed dysbiosis.In our studies, patients with lupus nephritis, especially the most severe cases, can develop expansions of single species and strains, also termed blooms.(A)A healthy gut microbiota community includes a dynamic equilibrium that is resilient following stressors such as nutritional and viral infections that generally reset to the preexisting balance.(B) Dysbiosis can be associated with inflammatory conditions and reflects shifts in the relative abundance of bacteria, reflecting metabolomic changes with some expansion of gut microbiota.(C) Severe dysbiosis can be associated with blooms that include large expansions of specific microbial species (e. g.R. gnavus in SLE) with a community diversity that is not resilient to stressors, though it is uncertain if blooms are causal in autoimmune diseases or reflect inflammation secondary to patients' underlying systemic disease states.Increased gut epithelial permeability secondary to host inflammatory and/or microbial metabolomic changes also occur.These depictions are oversimplifications representing the (A) balance vs. B) dysbiotic imbalances that occur in many patients with inflammatory and autoimmune diseases C) blooms of individual potential pathobionts, in the highly complex communities that reside within the human intestine.

Table 1 .
Recent reports on microbial dysbiosis in patients with rheumatic autoimmune disease